Fire resistant calcium sulphate-based products

ABSTRACT

The present invention provides calcium sulphate-based product having reduced shrinkage after exposure to high temperatures, the product comprising gypsum, a pozzolan source e.g. in an amount between 4-27 wt %) and a metal salt additive in an amount between 0.5 and 10 wt %). The pozzolan source may be selected from a kaolinitic clay material, fly ash, rice husk ash, diatomaceous earths, volcanic ashes and pumices, micro-silica, silica fume and silicone oil, The metal salt additive may be a metal salt which decomposes between a temperature of 300-500° C. to yield a metal oxide, e.g. magnesium nitrate.

This is a Continuation of U.S. application Ser. No. 15/525,467 filed May9, 2017, which issued as U.S. Pat. No. 10,662,113, which in turn is aNational Stage of PCT/EP2015/076785 filed Nov. 17, 2015, which claimsthe benefit of GB 1420766.6 filed Nov. 21, 2014. The disclosure of theprior applications is hereby incorporated by reference herein in itsentirety.

This invention relates to improved fire resistant calcium sulphate-basedproducts and, in particular, to calcium sulphate-basedbuilding/construction products having reduced shrinkage after exposureto high temperatures.

BACKGROUND

Calcium sulphate-based products are widely used in the construction ofbuildings, for example, to form internal partitions using wallboard,also known as dry wall, gypsum board or plaster board) and ceilings orto encase ducts e.g. ventilation ducts) within buildings.

Calcium sulphate-based products such as wallboard are typically formedby drying an aqueous slurry of the hemihydrate of calcium sulphateCaSO₄.½H₂O), also known as calcined gypsum or stucco, between two sheetsof lining paper or fibreglass matting. As the slurry dries and thecalcined gypsum is hydrated, a hard, rigid core of gypsum calciumsulphate dihydrate CaSO₄.2H₂O)) sandwiched between the liningsheets/mats is formed.

When wallboard is exposed to high temperatures such as those experiencedin a building fire, or those experienced by wallboards used for encasingducts carrying high temperature fluids, the water of crystallizationcontained within the gypsum is driven off to yield the anhydrite ofcalcium sulphate. Initially, this has the advantage that heat transferacross the wallboard is reduced thus helping to contain the heatemanating from a duct or generated during a building fire. However, attemperatures around 400-450° C., the initially formed AIII phaseanhydrite also known as γ-CaSO₄ or “soluble” anhydrite) converts to theAII phase or “insoluble” anhydrite) and this phase change results inshrinkage of the wallboard i.e. a loss of dimensional stability. Thisshrinkage which may be around 2% of the wallboard's length or width oraround 6 vol %) often causes the wallboards to pull away from theirsupporting structures. This is obviously undesirable. In situationswhere wallboard is used for internal partitions and a fire breaks out,shrinkage can leaves gaps exposing rooms adjacent to the fire source tothe effects of the heat/fire. Gaps also allow ingress of oxygen into thefire source thus fueling the fire and negating the effects of any firedoors.

At higher temperatures in excess of 600° C.), the insoluble anhydritegoes on to sinter resulting in large reductions in wallboard volume.This results in extreme shrinkage which eventually causes collapse ofthe internal walls/ceilings/duct casings as they are no longer held bytheir supporting structures.

Efforts have been made to improve the fire resistance of calciumsulphate-based products in an attempt to reduce shrinkage after exposureto high temperatures.

It is known e.g. from U.S. Pat. No. 2,526,066 and U.S. Pat. No.2,744,022, to add a combination of unexpanded vermiculite andnon-combustible fibres to the aqueous calcined gypsum slurry during themanufacture of wallboard.

During heat exposure the vermiculite contained within the wallboard coreexpands by an amount comparable to the amount of gypsum shrinkage thusresisting the shrinkage of the wallboard.

Wallboard containing unexpanded vermiculite and/or glass fibres hasfound extensive commercial excess.

U.S. Pat. No. 3,616,173 proposed adding small amounts preferably about2-5 wt %) of clay, colloidal silica or colloidal alumina to the gypsumcore in addition to the glass fibres and vermiculite.

The intention was to reduce the density of the fire resistant wallboard.Amounts greater than 20 wt % were found to result in a weak core thatdid not bind satisfactorily with the paper lining sheets.

US2003/0138614 discloses a fire resistant gypsum wallboard containing,in addition to unexpended vermiculite and glass fibres, 3-25 wt % of amineral additive which may be a day and 3-15 wt % hydrated alumina. Bestresults were achieved using 10-15 wt % of a day comprising 25 kaolinite.

U.S. Pat. No. 4,664,707 discloses a gypsum wall board made from a slurrycontaining glass fibres, calcium sulphate crystal fibres and 0.5-5 wt %clay, preferably a kaolinitic clay.

U.S. Pat. No. 6,569,541 discloses a water-resistant gypsum wallboardcontaining 5-15wt % of a mineral additive which may be a clay such askaolinite.

U.S. Pat. No. 5,985,013 discloses an ablative type heat protectingmaterial containing calcium sulphate hemihydrate and a hydrated salt, Anumber of hydrated salts are used including magnesium nitratehexahydrate used in an amount of 40 wt % based on the weight of dryingredients). The time taken for heat transfer across the heat ablativematerial was recorded. No mention is made of any effect on the shrinkageof the material after heating.

Calcium sulphate-based products are also used to cast metal or glassobjects. Calcium sulphate moulds are heated to 700-900° C. prior tobeing filled with molten metal/glass. It is important to control hightemperature shrinkage of such calcium sulphate-based moulds to ensurethat the moulds do not leak and to ensure that the cast metal/glassproducts are not warped.

A preferred aim of the present invention is to provide an improvedfire/heat resistant calcium-sulphate-based product having reducedshrinkage after heat exposure e.g. during a building fire. Such animproved fire resistant product may have particular use as a buildingproduct e.g. wallboard or panels for forming internal partitions inbuildings, ceiling tiles, wallboard or panels for encasingventilation/smoke extraction ducting, joint filler materials for joiningwallboard/panels/tiles or for moulds for use in metal/glass productcasting.

SUMMARY THE INVENTION

Accordingly, in a first aspect, the present invention provides a calciumsulphate-based product comprising gypsum, a pozzolan source and a metalsalt additive.

In a second aspect, the present invention provides a calciumsulphate-based product wherein the product is formed from drying anaqueous slurry containing calcined gypsum, a pozzolan source and a metalsalt additive.

In a third aspect, the present invention provides a method of forming acalcium sulphate-based product by drying an aqueous slurry comprisingcalcined gypsum, a pozzolan source and a metal salt.

In a fourth aspect, the present invention provides the use of acombination of a pozzolan source and a metal salt additive for reducingshrinkage during heat exposure of a calcium sulphate-based product,

In a fifth aspect, the present invention provides a calciumsulphate-based composition for use in forming a calcium sulphate-basedproduct by drying an aqueous slurry of the calcium sulphate-basedcomposition, the calcium sulphate-based composition comprising calcinedgypsum, a pozzolan source and a metal salt.

The present inventors have found that adding a combination of a pozzolansource and a metal salt results in a calcium sulphate-based productwhich maintains its dimensional stability even after heating up to1000°C. It is thought that a sintering process occurs which binds thegypsum together and helps improve the dimensional stability. Analysis ofthe product after heating and after the gypsum has been removed usingEDTA) shows that the pozzolan source forms an interlinking networkstructure which helps to bind the gypsum and thus increase stability.The presence of the metal salt reduces the temperature at which thepozzolan source transforms to the interlinking network structure andallows a reduction in the amount of pozzolan source needed. This may beas a result of the inclusion of the metal salt in the network structure.

The term “pozzolan source” is intended to include materials that arethemselves pozzolanic e.g. fly ash, rice husk ash, diatomaceous earth,volcanic ashes and pumices, micro-silica, silica fume) or which yield apozzolanic material upon heating e.g. a clay material such as akaolinitic clay material which yields metakaolin upon heating orsilicone oil which yields silica upon heating).

The term “silicone oil” is intended to refer to liquid polysiloxanes.The silicone oil may comprise a polydiorganosiloxane. The organo groupsmay be alkyl and/or aryl e.g. methyl and/or phenyl groups. An example ispolydimethylsiloxane PDMS). The silicone oil may comprise apolyorganohydrosiloxane. The organo group may be an alkyl or aryl groupe.g. a methyl and/or phenyl group. An example is polymethyihydrosiloxanePMHS). The silicone oil may comprise a copolymer of a diorganosiloxaneand an organohydrosiloxane or a blend of a polydiorganosiloxane and apolyorganohydrosiloxane.

The silicone oil may be anhydrous.

Preferably, the pozzolan source is a kaolinitic clay material ordiatomaceous earth.

The term “kaolinitic clay material” encompasses kaolinite Al₂Si₂O₅OH)₄),_(p)olymorphs of kaolinite such as dickite, halloysite and nacrite, ballclay which comprises 20-80% kaolinite, 10-25% mica, 6-65 quartz), fireclay and flint clay. An example of a suitable clay additive is Puroflo31™ manufactured by Sibelco and which comprises 66% kaolinite, 23% mica,6% feldspar and 1% quartz. The clay material is preferably anun-calcined clay material. Kaolinitic clay forms the pozzolanrnetakaolin during dehydration at high temperatures.

In the slurry used to form the calcium sulphate-based product and in thecalcium suiphate-based compositions, the pozzolan source may be providedhi an amount greater than 5 wt %, preferably between 5 wt % and 30 wt %,more preferably between 5 and 25 wt % e.g. between 5 and 10 wt %, andmost preferably between 5 and 9 wt % where wt % is based on the weightof the calcined gypsum, pozzolan source and metal salt).

In the calcium sulphate-based product, the pozzolan source may beprovided in an amount greater than 4 wt %, preferably between 4 and 27wt %, more preferably between 4 and 20 wt % and most preferably between4 and 9 wt % where wt % is based on the weight of the gypsum, pozzolansource and metal salt).

The metal salt is preferably a metal salt which decomposes between atemperature of 300-500° C. to yield a metal oxide.

The metal in the metal salt may be an alkaline earth metal e.g. calciumor magnesium. The metal may be a transition metal e.g. copper, zinc,iron. The metal may be aluminium. Preferably the metal is magnesium.

The salt may be a nitrate, a carbonate, a hydrogen carbonate, asulphate, a hydroxide or chloride. The salt may be hydrated.

Preferred metal salts are the nitrates of magnesium, copper, aluminium,calcium, zinc, and iron and magnesium chloride e.g. the hexahydrate).

In the slurry used to form the calcium sulphate-based product and in thecalcium sulphate-based composition, the metal salt may be provided in anamount greater than 1 wt %, preferably between 1 and 15 wt %, morepreferably between 1 and 10 wt % and most preferably between 2 and 9 wt%,

The pozzolan source and the metal salt may be included in the slurry andin the calcium-sulphate-based composition in a 1:1 wt % ratio where wt %is based on the weight of the calcined gypsum, pozzolan source and metalsalt). They may each be included in an amount of around 9 or 10 wt %.

In the calcium sulphate-based product, the metal salt may be provided inan amount greater than 0.5 wt %, preferably between 0.5 and 10 wt %,more preferably between 1 and 9 wt % and most preferably between 2 and 9wt % where wt % is based on the weight of the gypsum, pozzolan sourceand metal salt).

The term “gypsum” is intended to refer predominantly to calcium sulphatedihydrate CaSO₄.2H₂O).

The term “calcined gypsum” is intended to refer predominantly to calciumsulphate hemihydrate CaSO₄.½H₂O) but may also encompass any othercalcium sulphate compound having a lower bound water content thancalcium sulphate dihydrate e.g. calcium sulphate anhydrite).

In the slurry used to form the calcium sulphate-based product and in thecalcium sulphate-based composition, the calcined gypsum is preferablyprovided in an amount between 60 wt % and 95 wt %, more preferablybetween 75 and 95 wt % and most preferably between 75 and 90 wt % wherewt % is based on the weight of the calcined gypsum, pozzolan source andmetal salt).

In the calcium sulphate-based product, the gypsum is preferably providedin an amount between 65 wt % and 98 wt %, more preferably between 65 and90 wt % and most preferably between 65 and 85 wt % where wt % is basedon the weight of the gypsum, pozzolan source and metal salt).

In a particularly preferred embodiment, the calcium-sulphate basedproduct comprises 65 wt % -98 wt % gypsum, a pozzolan source and 0.5 wt%-9 wt % metal salt and may be formed from drying an aqueous slurrycontaining 60-95 wt % calcined gypsum, a pozzolan source and 1 wt % to 9wt % metal salt where wt % is based on the weight of the gypsum,pozzolan source and metal salt).

For this embodiment, the amounts and nature of the pozzolan source, thepreferred amounts of gypsum/calcined gypsum and the preferredamounts/nature of the metal salt may be as described above.

In another particularly preferred embodiment, the calcium-sulphate basedproduct comprises 65 wt %-98 wt % gypsum, a pozzolan source andmagnesium nitrate and may be formed from drying an aqueous slurrycontaining 60-95 wt % calcined gypsum, a pozzolan source and magnesiumnitrate where wt % is based on the weight of the gypsum, pozzolan sourceand magnesium nitrate).

For this embodiment, the amounts and nature of the pozzolan source, thepreferred amounts of gypsum/calcined gypsum and the preferred amounts ofthe magnesium nitrate may be as described above.

Preferably, the calcium-sulphate-based product contains substantially novermiculite. The present inventors have found that the addition of acombination of pozzolan source and metal salt can help minimiseshrinkage of a calcium-sulphate-based product e.g. gypsum wallboard evenin the absence of vermiculite.

In some embodiments, the calcium sulphate-based product containssubstantially no inorganic fibres e.g. no glass or asbestos fibres.

However, in some embodiments, the calcium sulphate-based product maycontain inorganic fibres e.g. glass fibres) and/or matting e.g. glassmatting) as this may help improve strength of the product prior toheating.

The calcium sulphate-based product may contain additives such asaccelerators, retarders, foaming/anti-foaming agents, fluidisers etc.,The accelerators may be, for example, freshly ground gypsum having anadditive of sugar or surfactant. Such accelerators may include GroundMineral NANSA GMN), heat resistant accelerator HRA) and ball milledaccelerator BMA). Alternatively, the accelerator may be a chemicaladditive such as aluminium sulphate, zinc sulphate or potassiumsulphate. In certain cases, a mixture of accelerators may be used, e.g.GMN in combination with a sulphate accelerator. As a furtheralternative, ultrasound may be used to accelerate the setting rate ofthe slurry, e.g. as described in US2010/0136259.

The term “calcium sulphate-based product” may include building materialssuch as wallboards with or without liners) with or without fibrousreinforcement), tiles e.g. ceiling tiles), duct encasement panels, jointfiller materials e.g. for joining adjacent wallboards/tiles/panelsetc.), plaster compositions or moulds for metal casting.

The term “calcium sulphate-based” will be readily understood as meaningthat the product comprises gypsum as a major component i.e. that gypsumis the largest single component in terms of wt % of the product. Theterm may mean that the product comprises gypsum in 40 wt %, 50 wt %, 60wt %, 70 wt %, 80 wt %, 90 wt % or greater based on the total weight ofthe product.

The calcium sulphate-based product may be a composite product e.g. itmay be a wallboard having a gypsum matrix core containing the shrinkageresistance additive) sandwiched between two liners e.g. paper liners orfibreglass matting).

Experimental

The following examples show products having reduced shrinkage at hightemperatures and are given by way of illustration only.

Control Sample 1—No Additives

200 g of calcined gypsum was added to 140 g of water at 40° C. and themixture was blended by hand for 30 seconds to form a slurry. The slurrywas poured into a cylindrical silicone mould height 25 mm, diameter 12mm) and the sample was dried at 40° C. overnight minimum 12 hours).

Control Sample 2—Kaolin 10 wt %)

180 g of calcined gypsum and 20 g of kaolin were dry blended and addedto 140 g of water at 40° C., The mixture was blended by hand for 30seconds to form a slurry. The slurry was poured into a cylindricalsilicone mould height 25 mm, diameter 12 mm) and the sample was dried at40° C. overnight minimum 12 hours).

Control Sample 3—Kaolin 30 wt %)

140 g of calcined gypsum and 60 g of kaolin were dry blended and addedto 140 g of water at 40° C. The mixture was blended by hand for 30seconds to form a slurry. The slurry was poured into a cylindricalsilicone mould height 25 mm, diameter 12 mm) and the sample was dried at40° C. overnight minimum 12 hours).

Control Sample 4—Magnesium Nitrate 9 wt %)

20 g of magnesium nitrate hexahydrate was added to 140 g of water at 40°C., 200 g of calcined gypsum was added to the waterimetal salt mixtureand the resulting mixture was blended by hand for 30 seconds to form aslurry. The slurry was poured into a cylindrical silicone mould height25 mm, diameter 12 mm) and the sample was dried at 40° C. overnightminimum 12 hours).

Control Sample 5—Copper Nitrate 7 wt %)

16 g of copper nitrate tetrahydrate was added to 140 g of water at 40°C. 200 of calcined gypsum was added to the water/metal salt mixture andthe resulting mixture was blended by hand for 30 seconds to form aslurry. The slurry was poured into a cylindrical silicone mould height25 mm, diameter 12 mm) and the sample was dried at 40° C. overnightminimum 12 hours).

Control Sample 6—Calcium Nitrate 8 wt %)

18 g of calcium nitrate tetrahydrate was added to 140 g of water at 40°C. 200 g of calcined gypsum was added to the water/metal salt mixtureand the resulting mixture was blended by hand for 30 seconds to form aslurry. The slurry was poured into a cylindrical silicone mould height25 mm, diameter 12 mm) and the sample was dried at 40° C. overnightminimum 12 hours).

Control Sample 7—Iron Nitrate 9 wt %)

20 g of iron (III) nitrate nonahydrate was added to 140 g of water at40° C. 200 g of calcined gypsum was added to the water/metal saltmixture and the resulting mixture was blended by hand for 30 seconds toform a slurry. The slurry was poured into a cylindrical silicone mouldheight 25 mm, diameter 12 mm) and the sample was dried at 40° C.overnight minimum 12 hours).

Control Sample 8—Aluminium Nitrate 9 wt %)

20 g of aluminium nitrate nonahydrate was added to 140 g of water at 40°C. 200 g of calcined gypsum was added to the water/metal salt mixtureand the resulting mixture was blended by hand for 30 seconds to form aslurry. The slurry was poured into a cylindrical silicone mould height25 mm, diameter 12 mm) and the sample was dried at 40° C. overnightminimum 12 hours).

Control Sample 9—Rice Husk Ash 10 wt %)

180 g of calcined gypsum was dry blended with 20 g of rice husk ash andthen blended by hand with 140 g of water for 30 seconds to form aslurry. The slurry was poured into a cylindrical silicone mould height25 mm, diameter 12 mm) and the sample was dried at 40° C. overnightminimum 12 hours).

Control Sample 10—Silicone Oil 10 wt %)

20 g of silicone oil was added to 140 g of water at 40° C. 200 g ofcalcined gypsum was added to the solution and was blended by hand for 30seconds to form a slurry. The slurry was poured into a cylindricalsilicone mould height 25 mm, diameter 12 mm) and the sample was dried at40° C. overnight minimum 12 hours).

The silicone oil used was SILRES® BS 94 provided by Wacker. This is ananhydrous silicone oil based on polyrethylhydrosiloxane.

Control Sample 11—Micro-Silica 10 wt %)

180 g of calcined gypsum was dry blended with 20 g of micro-silica andthen blended by hand with 140 g of water for 30 seconds to form aslurry. The slurry was poured into a cylindrical silicone mould height25 mm, diameter 12 mm) and the sample was dried at 40° C. overnightminimum 12 hours).

Control Sample 12—Diatomaceous Earth 10 wt %)

180 g of calcined gypsum was dry blended with 20 g of diatomaceous earthand then blended by hand with 140 g of water for 30 seconds to form aslurry. The slurry was poured into a cylindrical silicone mould height25 mm, diameter 12 mm) and the sample was dried at 40° C. overnightminimum 12 hours).

EXAMPLES

Sample formulations having the amounts of metal salt, pozzolan sourceand calcined gypsum shown in Table 1 below were prepared for all butExample 23) by mixing the metal salt with 140 g of water at 40° C. Thepozzolan source and calcined gypsum were dry blended and added to thewater/salt mixture. The resulting mixture was blended by hand for 30seconds to form a slurry. For example 23, the silicone oil and metalsalt were added to 140 g of water at 40° C. and then the calcined gypsumwas added to the solution to form a slurry which was blended by hand for30 seconds. Wt % amounts based on the dry ingredients) of each componentin the slurry are shown below in brackets.

The slurry was poured into a cylindrical silicone mould height 25 mm,diameter 12 mm) and dried overnight minimum 12 hours) at 40° C.

TABLE 1 Summary of sample formulations Pozzolan Metal Stucco/g source/gsalt/g Sample (wt %) (wt %) (wt %) Control 1 200  0  0 Control 2 180(90) 20 (10) Kaolin Control 3 140 (70) 60 (30)  0 Kaolin Control 4 200(91)  0 20 (9) Control 5 200 (93)  0 16 (7) Control 6 200 (92)  0 19 (8)Control 7 200 (91)  0 20 (9) Control 8 200 (91)  0 20 (9) Control 9 180(90) 20 (10)  0 Rice husk ash Control 10 180 (90) 20 (10)  0 Siliconeoil Control 11 180 (90) 20 (10)  0 Micro-silica Control 12 180 (90) 20(10)  0 Diatomaceous earth Example 1 180 (81.8) 20 (9.1) 20 (9.1) Mgnitrate (hexahydrate) Kaolin Example 2 140 (63.6) 60 (27.3) 20 (9.1) Mgnitrate (hexahydrate) Kaolin Example 3 180 (89.1) 20 (9.9)  2 (1.0) Mgnitrate (hexahydrate) Kaolin Example 4 195 (96.5)  5 (2.5)  2 (1.0) Mgnitrate (hexahydrate) Kaolin Example 5 195 (95.1)  5 (2.4)  5 (2.4) Mgnitrate (hexahydrate) Kaolin Example 6 180 (87.8) 20 (9.8)  5 (2.4) Mgnitrate (hexahydrate) Kaolin Example 7 190 (92.7) 10 (4.9)  5 (2.4) Mgnitrate (hexahydrate) Kaolin Example 8 180 (83.3) 20 (9.3) 16 (7.4) Cunitrate (tetrahydrate) Kaolin Example 9 140 (64.8) 60 (27.8) 16 (7.4) Cunitrate (tetrahydrate) Kaolin Example 10 180 (82.6) 20 (9.2) 18 (8.3) Canitrate (tetrahydrate) Kaolin Example 11 140 (64.2) 60 (27.5) 18 (8.3)Ca nitrate (tetrahydrate) Kaolin Example 12 180 (81.8) 20 (9.1) 20 (9.1)Fe (III) nitrate (nonahydrate) Kaolin Example 13  40 (63.6) 60 (27.3) 20(9.1) Fe (III) nitrate (nonahydrate) Kaolin Example 14 180 (81.8) 20(9.1) 20 (9.1) Al nitrate (nonahydrate) Kaolin Example 15 140 (63.6) 60(27.3) 20 (9.1) Al nitrate (nonahydrate) Kaolin Example 16 180 (81.8) 20(9.1) 20 (9.1) Zn nitrate (hexahydrate) Kaolin Example 17 140 (63.6) 60(27.3) 20 (9.1) Zn nitrate (hexahydrate) Kaolin Example 18 180 (89.1) 20(9.9)  2 (1.0) Mg chloride (hexahydrate) Kaolin Example 19 195 (96.5)  5(2.5)  2 (1.0) Mg chloride (hexahydrate) Kaolin Example 20 195 (95.1)  5(2.4)  5 (2.4) Mg chloride (hexahydrate) Kaolin Example 21 180 (87.8) 20(9.8)  5 (2.4) Mg chloride (hexahydrate) Kaolin Example 22 180 (82) 20(9) 20 (9) Mg nitrate (hexahydrate) Rice husk ash Example 23 180 (82) 20(9) 20 (9) Mg nitrate (hexahydrate) Silicone oil Example 24 180 (82) 20(9) 20 (9) Mg nitrate (hexahydrate) Micro-silica Example 25 180 (82) 20(9) 20 (9) Mg nitrate (hexahydrate) Diatomaceous earth

Linear shrinkage was measured using a Netzsch dilatometer. The sampleswere heated to 1000° C. at a rate of 5° C./min. The shrinkage wasmeasured in-situ using a transducer having a resolution of 8 nm.

The dilatometer results are shown in Table 2.

TABLE 2 Pozzolan Metal Stucco/ source/ salt/ wt % wt % wt % in slurry inslurry in slurry (wt % in (wt % in (wt % in Shrinkage (%) Sampleproduct) product) product) 500° C. 750° C. 900° C. 950° C. 1000° C.Control 1 100 0 0 −1.8 −3.6 −7.13 −18 Off scale Control 2 90 10 0 −1.59−2.50 −3.10 −5.11 −8.03 (91.4) (8.6) Control 3 70 30 0 −1.6 −2.7 −3.46−6.48 −8.59 (73.5) (26.5) Control 4 91 0 9 −0.02 −0.12 −0.17 −2.84 −6.51(92.2) (7.8) Control 5 93 0 7 −0.24 −1.54 −12/9 Off Off (93.7) (6.3)scale scale Control 6 92 0 8 −0.26 −2.84 −9.05 −9.08 −9.1 (92.6) (7.4)Control 7 91 0 9 −0.6 −0.7 −3.6 −7.7 −12.3 (92.2) (7.8) Control 8 91 0 9−0.7 −0.4 −0.4 −0.9 −3.5 (92.2) (7.8) Control 9 90 10 0 −1.7 −2.6 −3.5−7.7 −10.4 (91.4) (8.6) Control 10 90 10 0 −1.3 −2.2 −2.4 −3.2 −4.6(91.4 (8.6) Control 11 90 10 0 −2.0 −3.4 −4.8 −9.4 −11.7 (91.4) (8.6)Control 12 90 10 0 −1.8 −2.6 −3.5 −6.2 −8.5 (91.4) (8.6) Ex. 1 81.8 99.19.1 −0.13 0.02 0.04 −0.13 −0.69 Mg(NO₃)₂ (84.2) (7.9) (7.9) Ex. 2 63.627.3 9.1 −0.22 −0.23 −0.7 −1.52 −1.85 Mg(NO₃)₂ (67.5) (24.4) (8.1) Ex. 389.1 9.9 1.0 −1 −1.11 −1.36 −3.34 −6.07 Mg(NO₃)₂ (90.7) (8.5) (0.8) Ex.4 96.5 2.5 1.0 −1.25 −1.41 −1.42 −2.19 −12.4 Mg(NO₃)₂ (97.1) (2.1) (0.8)Ex. 5 95.1 2.4 2.4 −0.74 −0.61 −0.77 −1.3 −12.8 Mg(NO₃)₂ (95.9) (2.1)(2.1) Ex. 6 87.8 9.8 2.4 −0.18 0.022 0.16 −0.24 −1.45 Mg(NO₃)₂ (89.5)(8.4) (2.1) Ex. 7 92.7 4.9 2.4 −0.32 −0.068 0.13 −0.39 −6.95 Mg(NO₃)₂(93.8) (4.2) (2.1) Ex. 8 83.3 9.3 7.4 −0.19 −0.03 −0.29 −0.04 −1.49Cu(NO₃)₂ (85.6) (8.0) (6.4) Ex. 9 64.8 27.8 7.4 −0.47 −0.38 −0.52 −0.99−1.5 Cu(NO₃)₂ (68.6) (24.8) (6.6) Ex. 10 82.6 99.2 8.3 −0.3 −0.01 −0.3−0.24 −0.72 Cu(NO₃)₂ (84.9) (8.0) (7.2) Ex. 11 64.2 27.5 8.3 −0.49 −0.52−0.04 −0.52 −1.01 Cu(NO₃)₂ (68.0) (24.6) (7.4) Ex. 12 81.8 9.1 9.1 −0.75−0.64 −0.92 −1.0 −1.42 Fe(NO₃)₃ (84.2) (7.9) (7.9) Ex. 13 63.6 27.3 9.1−0.47 −0.34 −0.42 −0.68 −0.99 Fe(NO₃)₃ (67.5) (24.4) (8.1) Ex. 14 81.89.1 9.1 −0.78 −0.5 −0.32 −0.3 −0.36 Al(NO₃)₃ (84.2) (7.9) (7.9) Ex. 1563.6 27.3 9.1 −0.82 −0.93 −0.79 −0.80 −0.79 Al(NO₃)₃ (67.5) (24.4) (8.1)Ex. 16 81.8 9.1 9.1 −0.47 −0.31 −0.86 −2.07 −4.43 Zn(NO₃)₂ (84.2) (7.9)(7.9) Ex. 17 66 27.3 9.1 −0.05 0.1 −0.07 −0.6 −0.87 Zn(NO₃)₂ (67.5)(24.4) (8.1) Ex. 18 89.1 9.9 1.0 −0.5 −0.4 −0.07 −1.2 −1.4 MgCl₂ (90.7)(8.5) (0.8) Ex. 19 96.5 2.5 1.0 −0.58 −0.63 −2.64 −7.4 −8.69 MgCl₂(97.1) (2.1) (0.8) Ex. 20 95.1 2.4 2.4 −0.49 −1.45 −5.51 −8.89 −10.61MgCl₂ (95.9) (2.1) (2.1) Ex. 21 87.8 9.8 2.4 −0.3 −0.3 −0.5 −0.1 −0.1MgCl₂ (89.5) (8.4) (2.1) Ex. 22 82 9 9 −0.04 −0.04 −0.15 −1.85 −4.23Mg(NO₃)₂ (84.2) (7.9) (7.9) Ex. 23 82 9 9 −0.95 −1.42 −1.38 −1.52 −1.95Mg(NO₃)₂ (84.2) (7.9) (7.9) Ex. 24 82 9 9 −0.41 −0.31 −0.2 −0.7 −8.8Mg(NO₃)₂ (84.2) (7.9) (7.9) Ex. 25 82 9 9 −0.03 −0.01 −0.02 −0.07 −1.92Mg(NO₃)₂ (84.2) (7.9) (7.9)

The results show that a combination of pozzolan source and a metal saltcan help reduce shrinkage after exposure to elevated temperatures.Results are most pronounced when greater than 5 wt % of pozzolan sourceis used in the slurry greater than 4 wt % in the product) and whengreater than 1 wt % metal salt is used in the slurry greater than 0.5 wt% in the product). Results where an equal amount of pozzolan source andmetal salt are used are particularly pronounced.

The invention claimed is:
 1. A gypsum wallboard comprising: gypsum, apozzolan source and a metal salt additive, wherein: the gypsum isprovided in an amount of 40% or greater based on a total weight of thewallboard; the metal salt additive is a magnesium salt that decomposesbetween a temperature of 300-500° C. to yield a metal oxide; and thepozzolan source is selected from fly ash, rice husk ash, volcanic ash orpumice, micro-silica, silica fume, or clay material.
 2. The gypsumwallboard according to claim 1, wherein the pozzolan source is includedin an amount between 4 and 27 wt% (based on the weight of the gypsum,pozzolan source and metal salt additive).
 3. The gypsum wallboardaccording to claim 2, wherein the pozzolan source is included in anamount between 4 and 9 wt% (based on the weight of the gypsum, pozzolansource and metal salt additive).
 4. The gypsum wallboard according toclaim 1, wherein the metal salt additive is included in an amountbetween 0.5 and 10 wt% (based on the weight of the gypsum, pozzolansource and metal salt additive).
 5. The gypsum wallboard according toclaim 4, wherein the metal salt additive is included in an amountbetween 2 and 9 wt% (based on the weight of the gypsum, pozzolan sourceand metal salt additive).
 6. The gypsum wallboard according to claim 1,wherein the gypsum is provided in an amount between 65 and 98 wt% (basedon the weight of the gypsum, pozzolan source and metal salt additive).7. The gypsum wallboard according to claim 6, wherein the gypsum isprovided in an amount between 65 and 85 wt% (based on the weight of thegypsum, pozzolan source and metal salt additive).
 8. The gypsumwallboard according to claim 1, wherein a wt% of pozzolan source and wt%metal salt additive are equal.
 9. The gypsum wallboard according toclaim 1, wherein the metal salt additive is a nitrate, a carbonate, ahydrogen carbonate, a sulphate, a hydroxide or chloride.
 10. The gypsumwallboard according to claim 2, wherein the metal salt additive isincluded in an amount between 0.5 and 10 wt% (based on the weight of thegypsum, pozzolan source and metal salt additive).